why does testem need an empty testem.js file

I'm going to first apologize if I'm not articulating this correctly. I'm starting to run tests with testem and I've got it working by adding <script src="/testem.js" integrity=""></script> in my tests and an empty testem.js file in the test directory. I just don't know why testem needs it. I've searched everywhere and I couldn't find a good resource that explains this. Any help would be appreciated.